If a game does offer it, but doesn't let me play for long enough to decide whether I like the game before it starts serving up unskippable ads, I uninstall it.
Most games simply don't give enough time to decide if you even like it before they start throwing ads at you. That seems like a terrible policy in general, regardless of whether you have a "skip ads" IAP or not. You want to wait until the player has made up their mind about your game before you start degrading the experience. This seems like such an obvious thing I don't know why so many games are so bad about it.